gambler's den
A place where gamblers gather, often associated with illegal or underground gambling. The term is somewhat dated and may appear in period pieces or yakuza-related contexts.
Gamblers in the old days gathered at a gambler's den to gamble.
Compound of 盆 (bon, 'tray' or 'gambling board') and 中 (naka, 'inside'). The exact historical derivation is uncertain, but it likely refers to a place where gambling boards are set up.